The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In general, an exhaust system of an engine includes an exhaust gas after treatment apparatus such as a diesel oxidation catalyst (DOC), a diesel particulate matter filter (DPF), a selective catalyst reduction (SCR), a lean NOx trap (LNT), and the like in order to reduce carbon monoxide (CO), hydrocarbon (HC), particulate matters, nitrogen oxide (NOx), and the like, which are pollutants contained in the exhaust gas.
The DOC may oxidize all hydrocarbons and carbon monoxide in the exhaust gas. The DOC may also oxidize nitrogen monoxide to nitrogen dioxide.
The DPF may trap particulate matters in the exhaust gas, and purify the particulate matters through a chemical conversion process.
Further, the SCR converts a reducing agent (urea), which is injected in the direction of the exhaust gas stream by an injector, into ammonia (NH3) using heat of the exhaust gas, and reduces nitrogen oxide to nitrogen gas (N2) and water (H2O) by a catalyst reaction between nitrogen oxide in the exhaust gas and ammonia in the presence of an SCR catalyst.
However, we have discovered that in a high temperature and a high load condition, the NOx absorption efficiency of the LNT catalyst is deteriorated since there is a limit to the NOx purification performance.
Particularly, we have found that the temperature of the catalyst is increased more than about 400° C. in the high temperature and the high load condition and a large amount of NOx is flowed into the catalyst. Therefore, the NOx is not sufficiently absorbed by the LNT catalyst and exhausted.